Fig. 1. Pain test results obtained in 12 healthy volunteers before, during (steps 1–4), and after (postinfusion 1–2) a steady state infusion of saline placebo, dexmedetomidine, and alfentanil are depicted as the median and interquartile range. Stable dexmedetomidine and alfentanil plasma concentrations were achieved during four geometrically increasing infusion steps (  bottom graph  ). Alfentanil, but not dexmedetomidine, increased the heat and electrical pain threshold and pain tolerance compared with placebo. A significant relationship was detected between the alfentanil plasma concentration and the heat pain tolerance, the electrical pain threshold and the electrical pain tolerance.